1. Field of the Invention
The present invention relates to shock absorbers. More particularly, the invention relates to a damper for a shock absorber that meters fluid while reducing the effects of cavitation typically associated with annular flow paths of fluid through metering systems. More particularly still, the invention relates to reducing cavitation and the effects of cavitation in a front fork assembly for a two wheeled vehicle.
2. Description of the Related Art
An integral shock absorber usually consists of two parts: a spring and a damper. The spring may include a steel or titanium coil, an elastomer, or even compressed gas. A typical hydraulic damper creates a damping effect by inhibiting the flow of oil through specific regions of the device, such as a piston that moves in a dampening chamber.
Dampers are used in vehicle front and rear shock absorbers. FIG. 1 shows a front fork 100 for a two wheeled vehicle such as a bicycle (not shown). The fork 100 includes two telescopic tubes 105, 110, and a brace 115 to help keep the tubes parallel to one another in use.
In a typical fluid operated damper, the damping fluid flow is inhibited by forcing fluid through a restrictive area or orifice, which effectively slows the movement of the damper during the compression and rebound strokes. In one arrangement, the fluid is inhibited as it moves through a needle valve having a restricted annular area formed between a smooth tapered needle member and a smooth seat. As the fluid moves through the restriction, the fluid velocity increases substantially and a corresponding pressure drop occurs across the needle valve. If the pressure of a liquid damping fluid is reduced below the fluid vapor pressure of that fluid (e.g. oil), by high velocity flow for example, vapor bubbles will form and then collapse when they subsequently enter a region of pressure above the fluid vapor pressure. Cavitation, or the formation and collapse of bubbles in such a reduced pressure area, may cause noise and/or damage to the surrounding part surfaces. Such cavitation depends on the initial fluid pressure and the amount of pressure reduction. At higher initial fluid pressures cavitation is less likely because a larger pressure reduction is required to reach fluid vapor pressure (which is a constant at constant temperature). Cavitation noise can be undesirable in certain applications such as on a bicycle. A bicycle rider can encounter many successive bumps in a short amount of time. In this situation the rider must concentrate intently to avoid crashing. The repetitive noise coming from the damper can cause a distraction which could potentially cause the rider to lose concentration and subsequently lose control of the bicycle. Additionally, the noise can be an annoyance as bicycles are typically very quiet. Such noise can also detract from a user's perception of quality or robustness of a bicycle and associated components.
Dampers may also be used on machines in factories to control machine motion and vibration. In a factory setting a damper may be installed on a cyclically operating machine. Workers may have to work near the machine for hours in succession and the noise could potentially cause damage to a worker's ears. Moreover, the collapsing of bubbles can generate extremely high localized energy which can damage part surfaces within the damper. Cavitation damage and “explosive” cavitation damage (referring to the sudden implosion of cavitation bubbles) to fluid adjacent parts are well documented. Thus, a simple solution for reducing cavitation or the magnitude of cavitation events in a damper is very desirable.